Intraspecific body size variation in spiders is mediated by multiple stressors in an urban area

The urbanization process causes multiple environmental changes, encompassing habitat fragmentation, increasing tem- perature and variation in food availability, imposing significant challenges to living organisms. The synergistic effect of these stressors may have repercussions on species traits, especially on those strictly related to species survival, such as body size. We investigated the intraspecific body size variation along an urbanization gradient, considering multiple envi- ronmental facets in the city of Torino (NW-Italy), in three spider species: the sedentary cobweb spider Asagena italica (Araneae Theridiidae), the cursorial wolf spider Pardosa proxima (Araneae, Lycosidae) and the ballooning money spider Erigone autumnalis (Araneae, Linyphiidae). Our findings revealed distinct patterns among the examined species, probably related to the species’ intrinsic ability to deal with environmental changes occurring in the urban environment. The seden- tary cobweb spider responded at the local scale, exhibiting body size reduction at increasing temperature due to increased metabolic costs, and body size increase at both increasing vegetation cover and species richness of the spider community, probably reflecting variation in food availability. Conversely, the cursorial wolf spider displayed a significant body size increase at decreasing average community body size, as a possible reduction of intraguild predation and competition for food. The highly dispersive money spider responded at landscape scale showing smaller body sizes at increasing habitat fragmentation, possibly due to the enhanced dispersal ability of smaller individuals. As spiders are predators exerting a significant top-down control on invertebrate communities, these complex findings underscore the need for further investi- gation into how intraspecific body size variation of spiders along urbanization gradients may affect food webs. Importantly, our results highlight that species do not respond uniformly to urban-driven environmental pressures; rather, their responses are strongly mediated by species-specific ecological traits such as mobility, dispersal strategy, and foraging behavior. Understanding these trait-based responses is crucial for predicting the broader ecological consequences of urbanization.